1. Field of the Invention
The disclosed invention relates to a method and apparatus for providing full torque dynamic braking of induction motors over the full range of operating speed.
2. Description of the Prior Art
Although direct current series motors have usually been used for vehicle traction drives because of their desirable speed-torque characteristics and adaptability to dynamic braking, the alternating current, squirrel-cage induction motor is inherently of very rugged construction and would be highly suitable for the severe conditions of traction service in which a propulsion motor is subjected to severe vibration and mechanical shock conditions as well as being exposed to dirt and other airborne contamination. The development of high power, variable frequency static inverters, has made it possible to consider the use of induction motors for traction drives in order to take advantage of their rugged construction, freedom from commutation problems and relative ease of control. One problem, however, in the use of induction motors in traction drives has been that of obtaining adequate braking torque when operating in the braking mode. Since the torque developed in the motor is a function of the voltage applied to the stator windings, substantially constant torque with increasing speed may be obtained in the motoring operation by increasing the applied voltage for the inverter approximately proportionally with inverter output frequency until the maximum voltage rating of the inverter is reached. After the maximum voltage rating of the inverter is reached, the torque decreases as the frequency and speed increase. Although this is generally acceptable for motor operation because less accelerating capability is required at the higher speeds, for braking operation it is desirable to have a braking torque at least as high as the maximum motor torque over the entire range of operating speeds. That is, the machine should be operated at constant torque even at speeds where the induction machine voltage exceeds the inverter voltage so as to maintain essentially constant braking torque and provide a constant rate of deceleration over a wide speed range.
Since the torque developed by the machine is a function of the voltage across the stator windings, maximum braking torque over a wide speed range could be supplied merely by increasing inverter voltage. However, this would require an inverter whose maximum voltage rating is several times the maximum applied voltage needed during motor operation so that the size and cost of the inverter would make it impractical for traction use. Therefore, induction machine braking circuits of the prior art have provided for increasing the voltage across the motor windings during the braking operation by inserting a braking resistance in series with the induction machine windings. Since the torque developed in the machine is a function of a voltage across the machine windings, this results in increased torque without requiring any increased voltage supplied by the inverter to the machine windings above that required for motor operation. The desired braking torque can thus be obtained without any increase in size or cost of the inverter above that required for motoring operation. Braking torque is controlled to obtain a desired braking characteristic or rate of deceleration by controlling the effective value of the braking resistance during the braking operation through phase control of firing pulses applied to thyristors which shunt the braking resistance. Such a method for controlling induction motor braking torque by controlling the effective resistance of braking resistors in series relation with the induction motor windings is disclosed in U.S. Pat. No. 3,815,002 of S. Clemente and B. R. Pelly.
The prior art methods for increasing the dynamic braking capability of an induction machine variable frequency drive combination have performance characteristics which are limited by the effective inductance of the braking resistors and the inductance of the motor. Since improved performance of these prior art methods required non-reative braking impedance elements, capacitors were used to compensate for the inductance of the braking resistors and to partially compensate for the inductance of the motor windings and the current phase lag caused by the controlled conduction of the braking thyristors. This capacitive compensation significantly improves the performance of the braking scheme, particularly at higher operating frequencies. It was recognized that it would be advantageous to significantly reduce the capacitance required in induction motor dynamic braking circuits of the prior art. Additionally, because the wye connected resistors of induction motor dynamic braking circuits of the prior art required that each leg of the wye connected resistors be individually balanced to achieved line-to-line voltages, it was also recognized that it would be advantageous to provide for motor voltages which are inherently balanced.